1. Field of the Invention
The present invention generally relates to filtration of thermoplastic polymer melt and more particularly to continuous filtering of molten polymer melt which allows for changing of filters without interrupting polymer flow.
2. Related Background Art
It is common to filter melted plastic in a high pressure line leading to an extrusion die to insure high quality of the extruded products and avoid plugging openings due to the presence of contaminants in the melt.
Melted plastic presents unique problems for standard fluid filtering techniques by virtue of its substantial viscosity, the high temperatures at which the plastic must be maintained to insure satisfactory flow yet without high temperature degradation, the extremely high pressures to which the plastic is subjected to cause the flow, and the need for homogenous flow that not only is devoid of contaminants but of air and other gases as well.
Thermoplastic polymer is very often formed into a finished or semi-finished state by an extrusion process in which the polymer and optional other ingredients are mixed in melt form and forced through an opening of suitable dimension in a die into a mold or into the air as a sheet or strand. When it is required that the polymer be of high quality it is usually necessary to filter the extruding melt to remove unwanted matter, such as gel particles, incompletely dispersed additives compounded therewith and burnt stock from local overheating and degradation of the polymer. Gel particles are regions of higher than average viscosity due to excess molecular weight or cross-linking of polymer molecules. The presence of contaminants is less with prime virgin plastic but increased when plastic trim, rejects and start-up/shut-down scrap is included in the polymer melt stream.
Prior art filters commonly comprise filter screens interposed in the path of the melt. Such apertures are selected based on the size of contaminant being removed from the melt flow. The screen blocks contaminant material from passage through the screen openings and allows contaminant-free melt to pass freely to the outlet.
Features desired in a molten polymer filter system include: continuous filtration without interruption of melt flow as a blocked filter element is replaced with a clean one; and minimum "dead spot" areas for polymer to cease flowing locally while the main flow continues. This can result in overheating a heat sensitive polymer causing degradation and formation of burnt off-grade material.